Numerical Evaluation of Tunnel Portal Slope Stability at

Journal of Applied Geology, vol. 5(1), 2020, pp. 40–52DOI: http://dx.doi.org/10.22146/jag.57044

Numerical Evaluation of Tunnel Portal Slope Stability at Bagong Dam Site,East Java, Indonesia

Irien Akinina Fatkhiandari, I Gde Budi Indrawan*, and Dwikorita Karnawati

Department of Geological Engineering, Faculty of Engineering, Universitas Gadjah Mada, Yogyakarta, Indonesia

ABSTRACT. Geometries of excavated tunnel portal slopes at Bagong Dam site was initiallydesigned without taking into account the earthquake load. The excavated slope designsalso assumed that the rocks comprising the slopes were homogenous. The purpose of thisresearch was to evaluate the stability of the excavated tunnel inlet and outlet slopes at theBagong Dam site under static and earthquake loads using the finite element method. Thestability of the natural slopes was also analyzed for comparison. The numerical static andpseudostatic analyses of slope stability were carried out using RS2 software (Rocscience,Inc.). Input data used in the numerical analyses were obtained from engineering geolog-ical mapping, rock core analyses, and laboratory tests. The seismic coefficient applied inthe pseudo-static slope stability analyses was determined following guideline describedin Indonesian National Standard. The engineering geological mapping and evaluation ofrock cores indicated that the inlet tunnel slope consisted of four types of materials, namelyresidual soil, poor quality of volcanic breccia, very poor quality of volcanic breccia, andgood quality of volcanic breccia. The outlet portal slope consisted of six types of materi-als, namely residual soil, very poor quality of limestone, poor quality of limestone, verypoor quality of volcanic breccia, poor quality breccia, and good quality breccia. Based onthe secondary elastic wave velocity (Vs) values, the rock masses in the research area wereclassified as hard rock (SA). Seismic analyses based on the earthquake hazard source mapwith a 10 % probability of exceedance in 50 years provided by the National EarthquakeCenter (2017) indicated that the PGA and the corresponding amplification factor FPGA inthe research area were 0.3 and 0.8, respectively. The calculated seismic coefficient for thepseudostatic slope stability analyses was 0.12. The numerical analysis results showed that,in general, earthquake load reduced critical Strength Reduction Factor (SRF) values of theslopes. However, the natural and excavated tunnel portal slopes were relatively stable un-der static and earthquake loads. The natural slope at the tunnel inlet with a 40° inclinationhad a critical SRF value of 4.0, while that of at the tunnel outlet with a 51° inclination hada critical SRF value of 2.6. Under static load, the excavated slopes at the tunnel inlet andoutlet having a 45° inclination had critical SRF values of 2.4 and 5.0, respectively. Underearthquake load, the excavated slopes at the tunnel inlet and outlet had critical SRF valuesof 2.3 and 3.5, respectively.

Keywords: Bagong dam · Finite element method · GSI · RS2 · Slope stability.

*Corresponding author: I G. B. INDRAWAN, De-partment of Geological Engineering, Universitas GadjahMada. Jl. Grafika 2 Yogyakarta, Indonesia. E-mail:[email protected]

1 INTRODUCTION

The Bagong Dam site is located at Sumurup andSengon Villages, Bendungan District, Treng-galek Regency, East Java Province (Figure 1).For the construction of the earth-fill dam, theriver water was planned to be diverted using a

2502-2822/© 2020 The Authors. Open Access and published under the CC-BY license.

http://dx.doi.org/10.22146/jag.57044

mailto:[email protected]

NUMERICAL EVALUATION OF TUNNEL PORTAL SLOPE STABILITY AT BAGONG DAM SITE

diversion tunnel. Geometries of the inlet andoutlet tunnel portal slopes were designed byBBWS Brantas (2017) using a numerical soft-ware Plaxis developed by Bentley Systems, Inc.However, earthquake load was not consideredinto the slope designs. In addition, the rockscomprising the slopes were assumed to be ho-mogenous, and the shear strength parametersfor the slope stability analyses were obtainedfrom laboratory direct shear strength tests. Aslocated in an active seismic region, the stabilityof the tunnel portal slopes may be affected byearthquakes. As discontinuities tend to reducerock mass strength, ignoring rock mass qualitycontrolled by rock fractures may lead to over-estimated slope factor of safety. Therefore, sta-bility evaluation of the designed inlet and por-tal slopes by taking into account the earthquakeload and rock fracturing degree is necessary.

The finite element method has been increas-ingly used in slope stability analyses (Hammahet al., 2004). Excellent reviews of finite elementanalyses of slope stability are provided in nu-merous textbooks (e.g., Duncan et al., 2014; Wyl-lie, 2018). One of the advantages of the finite el-ement method over the traditional limit equilib-rium method is that no assumption needs to bemade in advance about the shape or location ofthe failure surface (e.g., Griffith and Lane, 1999;Rocscience Inc., 2001).

This paper presents results of engineering ge-ology study carried out to evaluate the stabilityof designed tunnel portal slopes at The BagongDam site under static and earthquake loads us-ing the finite element method. Results of theengineering geological mapping and rock coreevaluations are presented, and results of staticand pseudostatic slope stability analyses arehighlighted.

2 GEOLOGICAL SETTING

The Regional Geological Map of Madiun Sheetprepared by Hartono et al. (1992) shows thatthe Bagong Dam site and the surrounding con-sist of the Mandalika and Wonosari Formations(Figure 2). The Mandalika Formation was es-timated to form in Oligocene to Early Miocene,while the Wonosari Formation was estimated toform in Early to Late Miocene. Based on sur-face geological mapping and evaluation of drillcores, Fatkhiandari (2020) indicated that two

main types of lithologies, namely volcanic brec-cia and limestone, existed in the Bagong Damsite were likely members of the Mandalika andWonosari Formations, respectively. In addition,faults and joints observed in the Bagong Damsite and the surrounding had a consistent ori-entation to the regional Meratus structural pat-tern (NE–SW), as described by Pulunggono danMartodjodjo (1994).

3 METHOD

Engineering geological mapping around thetunnel portals, evaluation of rock cores at fourboreholes drilled by BBWS Brantas (2014) nearthe inlet and outlet tunnel portals (Figure 1),and laboratory testing of soil and rock sampleswere carried out to obtain data of rock masslayers and the engineering properties. Rockmass quality was determined by the GeologicalStrength Index (GSI) of the rock cores followingequation proposed by Hoek et al. (2013) as fol-lows:

GSI = 1.4Jcond +RQD

2(1)

where Jcond = joint conditions of rock coresas described in Bieniawski (1989); RQD = rockquality designation.

The pseudostatic slope stability analyseswere carried out to estimate the stability ofthe tunnel portal slopes under an earthquakeload. Following procedures described in SNI8460 (BSN, 2017), the seismic coefficient usedin pseudostatic slope stability analyses werecalculated as 0.5 of the peak ground accelera-tion (PGA) determined by considering the siteclass and amplification factor (FPGA). In addi-tion, the SNI 8460 (BSN, 2017) adopts groundmotions with a 10 % probability of exceedancein 50 years (corresponding approximately toa 500-year return period) for seismic slopedesign. The peak ground acceleration at par-ticular sites can be obtained from earthquakehazard source maps produced by the NationalEarthquake Center (2017).

The static and pseudostatic slope stabilityanalyses were carried out using a finite elementbased RS2 software developed by Rocscience,Inc. The input parameters in the slope stabilityanalyses are shown in Table 1. Figure 3 showsthe geometries of the natural slopes with the

Journal of Applied Geology 41

FATKHIANDARI et al.

FIGURE 1. Research area at the Bagong Dam site.

42 Journal of Applied Geology


FIGURE 2. Regional geological map of the Madiun sheet (Hartono et al.,1992).

rock masses applied in the slope stability anal-yses. The natural slope at the tunnel inlet had a40° inclination, while that of at the tunnel outlethad a 51° inclination. The groundwater levelsin both slopes were inferred from drilling data.Following the best practice of excavated slopedesign, the excavated slopes at the tunnel in-let and outlet were designed to have a 1:1 sloperatio (or 45° inclination). The SNI 8460 (BSN,2017) required a minimum factor of safety val-ues of 1.5 and 1.1 for permanent slopes understatic and earthquake loads, respectively. Inthe numerical finite element analyses, the fac-tor of safety values was represented by criticalStrength Reduction Factor (SRF) values.

4 RESULTS AND DISCUSSION

4.1 Rock mass types and propertiesTypes and qualities of the rock masses compris-ing the inlet and outlet tunnel portal slopes areshown in Table 2. Those characteristics of therock masses were determined from the engi-neering geological mapping and evaluation ofthe rock cores at Borehole BT3 and BBT 3 (Ta-ble 1). The inlet tunnel slope consisted of fourtypes of materials, namely residual soil, poorquality of volcanic breccia, very poor quality ofvolcanic breccia, and good quality of volcanicbreccia. Meanwhile, the outlet portal slope con-

sisted of six types of materials, namely residualsoil, very poor quality of limestone, poor qual-ity of limestone, very poor quality of volcanicbreccia, poor quality breccia, and good qualitybreccia. Typical cores of rock masses compris-ing the slopes are shown in Figure 4. The ma-terial properties from the laboratory tests (Ta-ble 4) then used as input parameters in the slopestability analyses. Intact rock constant (mi) ofeach rock mass quality was estimated based onthe range of mi value for each rock type pro-vided in the RS2 database by considering theweathering degree of the intact rock. Distur-bance factor (D) values were all set into zero byassuming that the excavation processes causedminimum disturbance to the surrounding rockmasses.

4.2 Earthquake loadMeasurement data of the previous study(BBWS Brantas, 2014) indicated that the re-search area had more than 1500 m/s secondaryelastic wave velocity (Vs) values. Therefore,the rock masses are classified as a hard rock(SA) class. The earthquake hazard source mapwith a 10 % probability of exceedance in 50years provided by the National EarthquakeCenter (2017) indicated that the PGA and thecorresponding amplification factor FPGA in


FATKHIANDARI et al.

TABLE 1. Input parameters for slope stability analyses.

Analysis type Plane strain

Stress analysis Tensile failure reduce shear strength;Joint tension reduce joint stiffnessEffective stress analyses (static load); total stress analyses (earthquake load)

Mesh Graded, 3-noded triangle

Field stress Gravity, actual ground surface

Displacement Top: FreeSide: Restrain XYBottom: Restrain XY

Material properties Initial element loading: Field stress & body forceFailure criteria: Mohr-Coulomb (soil); Generalized Hoek-Brown (rock)Material type: Plastic

TABLE 2. Rock masses at borehole BT3 (inlet slope).

Depth (m) Material Weathering degree Jcond RQD GSI Rock mass quality

0–3 Residual soilCompletelyweathered 0 0 - -

3.5–30 Volcanic breccia Moderatelyweathered 15 7 26

Poor quality of volcanicbreccia

30–40 Volcanic breccia Highly weathered 10 0 15Very poor quality ofvolcanic breccia

40–70 Volcanic breccia Slightly weathered 25 44 59.5Good quality ofvolcanic breccia

TABLE 3. Rock masses at borehole BBT3 (outlet slope).

Depth (m) Material Weathering degree Jcond RQD GSI Rock mass quality

0–3 Residual soilCompletelyweathered 0 0 - -

3–17 Limestone Highly weathered 10 0 15Very poor quality oflimestone

17–20.8 Volcanic breccia Moderatelyweathered 15 7 26

Moderate quality ofvolcanic breccia

20.8–35.2 Limestone Very highlyweathered 10 6 18

Very poor quality oflimestone

35.2–50 Volcanic breccia Slightly weathered 25 70 72.5Good quality ofvolcanic breccia



1

1

Residual soil

Poor quality breccia

Very poor quality breccia

Good quality breccia

60

40

20

0

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Analysis Description

CompanyScale 1:509Drawn By

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1

Residual soil

Poor quality of limestone

Poor quality of volcanic breccia

Very poor quality of limestone

Good quality of volcanic breccia

Very poor quality of volcanic breccia

12

51

00

75

50

25

0-2

5

0 25 50 75 100 125 150 175 200 225 250 275



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FIGURE 3. Geometries of natural slopes: (a) inlet; (b) outlet.


FATKHIANDARI et al.

C

A B

D

FIGURE 4. Photographs of rock cores: (A) very poor quality of limestone; (B) very poor quality of volcanicbreccia; (C) moderate quality of volcanic breccia; (D) good quality of volcanic breccia.

the research area were 0.3 and 0.8, respectively.The calculated seismic coefficient for the pseu-dostatic slope stability analyses was, therefore,0.12.

4.3 Portal slope stabilityContours of maximum shear strains resultedfrom the slope stability analyses for the natu-ral and excavated slopes under static and earth-quake loads are shown in Figure 5 to Figure 8.At the same time, the critical SRF values aresummarized in Table 5. In general, the naturaland designed excavated slopes met the stabilityrequirements specified in SNI 8460 (BSN, 2017),where the slopes under static and earthquakeloads had critical SRF (or factor safety) of morethan 1.5 and 1.1, respectively. Figure 6 and Fig-ure 8 show that maximum shear strains devel-oped at the toes of the inlet and outlet excavatedslopes above the tunnel portal. The maximumshear strain values were relatively insignificant(i.e., less than 5 %), and the critical SRF valueswere relatively high.

The simulation results imply that the earth-quake load reduces the critical SRF values ofall the slopes. Under the static load, the criti-cal SRF value of the inlet slope decreased as theslope inclination increased due to excavation

(i.e., 40° for natural inlet slope and 45° for exca-vated inlet slope). Meanwhile, the critical SRFvalue of the outlet slope increased as the slopeinclination reduced due to excavation (i.e., 51°for natural outlet slope and 45° for excavatedoutlet slope). The trends of decreased and in-creased critical SRF values of the inlet and out-let slopes, respectively, under static load, wereconsistent with those under earthquake load(Table 5).

5 CONCLUSIONS

The engineering geological mapping and eval-uation of rock cores indicated that the inlet tun-nel slope consisted of four types of materials,namely residual soil, poor quality of volcanicbreccia, very poor quality of volcanic breccia,and good quality of volcanic breccia. The out-let portal slope consisted of six types of mate-rials, namely residual soil, very poor quality oflimestone, poor quality of limestone, very poorquality of volcanic breccia, poor quality brec-cia, and good quality breccia. Based on thesecondary elastic wave velocity (Vs) values, therock masses in the research area were classi-fied as hard rock (SA). Seismic analyses basedon the earthquake hazard source map with a10 % probability of exceedance in 50 years pro-



1

1

Critical SRF: 3.99

MaximumShear Strain

0.00

0.08

0.16

0.24

0.32

0.40

0.48

0.56

0.64

0.72

0.80

60

40

20

0

0 20 40 60 80 100 120 140



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Critical SRF: 2.71

0.12

MaximumShear Strain

0.00

0.30

0.60

0.90

1.20

1.50

1.80

2.10

2.40

2.70

3.00

60

40

20

0

0 10 20 30 40 50 60 70 80 90 100 110 120 130



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FIGURE 5. Contours of maximum shear strains developed in natural inlet slopes resulted from (a) static; (b)pseudostatic analyses.


FATKHIANDARI et al.

1

1

Critical SRF: 2.4MaximumShear Strain

0.00

0.01

0.01

0.02

0.03

0.04

0.04

0.05

0.06

0.06

0.07

60

40

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0 10 20 30 40 50 60 70 80 90 100 110 120 130



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Critical SRF: 2.31

0.12

MaximumShear Strain

0.00

0.08

0.16

0.24

0.32

0.40

0.48

0.56

0.64

0.72

0.80

60

50

40

30

20

10

0

0 10 20 30 40 50 60 70 80 90 100 110



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FIGURE 6. Contours of maximum shear strains developed in excavated inlet slopes resulted from (a) static;(b) pseudostatic analyses.



1

1

Critical SRF: 2.63

MaximumShear Strain

0.00

0.00

0.00

0.00

0.00

0.01

0.01

0.01

0.01

0.01

0.01

150

125

100

75

50

25

0

0 25 50 75 100 125 150 175 200 225 250 275



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Critical SRF: 2.01

0.12

MaximumShear Strain

0.00

0.00

0.00

0.00

0.00

0.01

0.01

0.01

0.01

0.01

0.01

15

01

25

10

07

55

02

50

0 25 50 75 100 125 150 175 200 225 250 275



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FIGURE 7. Contours of maximum shear strains developed in natural outlet slopes resulted from (a) static; (b)pseudostatic analyses.


FATKHIANDARI et al.

1

1

Critical SRF: 4.97

MaximumShear Strain

0.00

0.01

0.01

0.02

0.03

0.04

0.04

0.05

0.06

0.06

0.07

125

100

75

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0

0 25 50 75 100 125 150 175 200 225 250 275



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Critical SRF: 3.49

0.12

MaximumShear Strain

0.00

0.01

0.01

0.01

0.02

0.03

0.03

0.03

0.04

0.05

0.05

17

51

50

12

51

00

75

50

25

0

0 25 50 75 100 125 150 175 200 225 250 275

FIGURE 8. Contours of maximum shear strains developed in excavated outlet slopes resulted from (a) static;(b) pseudostatic analyses.


NUMERICAL EVALUATION OF TUNNEL PORTAL SLOPE STABILITY AT BAGONG DAM SITETA

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TABLE 5. Summary of slope stability analysis re-sults.

Slope Critical SRF

Natural slope Excavated slope

Staticload

Earth-quakeload

Staticload

Earth-quakeload

Inlet 4.0 2.7 2.4 2.3Outlet 2.6 2.0 5.0 3.5

vided by the National Earthquake Center (2017)indicated that the PGA and the correspondingamplification factor FPGA in the research areawere 0.3 and 0.8, respectively. The calculatedseismic coefficient for the pseudostatic slopestability analyses was 0.12. The numerical anal-ysis results showed that the earthquake loadreduced the critical Strength Reduction Factor(SRF) values of the slopes. However, the naturaland excavated tunnel portal slopes were rela-tively stable under static and earthquake loads.The natural slope at the tunnel inlet with a 40°inclination had a critical SRF value of 4.0, whilethat of at the tunnel outlet with a 51° inclina-tion had a critical SRF value of 2.6. Under staticload, the excavated slopes at the tunnel inletand outlet having a 45° inclination had criti-cal SRF values of 2.4 and 5.0, respectively. Un-der earthquake load, the excavated slopes atthe tunnel inlet and outlet had critical SRF val-ues of 2.3 and 3.5, respectively. In summary,the natural and excavated inlet and outlet por-tal slopes met the requirements specified in SNI8460 (BSN, 2017) for seismic slope designs.

ACKNOWLEDGEMENTS

The authors would like to thank Balai BesarWilayah Sungai (BBWS) Brantas, the Ministryof Public Works and Housing of Indonesia, forpermission conducting this research. The firstauthor would like to thank the Ministry of Pub-lic Works and Housing of Indonesia for themaster’s degree scholarship. The assistance ofMs. Elisabeth Cary Cesaria in conducting fieldand laboratory works is gratefully acknowl-edged.

REFERENCES

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Bieniawski, Z.T. (1989) Engineering Rock Mass Clas-sifications. John Wiley and Sons.

BSN (2017) SNI (Standar Nasional Indonesia) 8460:Persyaratan Perancangan Geoteknik. Jakarta.

Duncan, J.M., Wright, S.G., and Brandon, T.L. (2014)Soil Strength and Slope Stability. John Wiley &Sons, Inc.

Fatkhiandari, I.A. (2020) Analisis Kestabilan danSistem Penyangga Terowongan Saluran Penge-lak Bendungan Bagong Kabupaten TrenggalekProvinsi Jawa Timur. Master Thesis. Departe-ment of Geological Engineering Universitas Gad-jah Mada. Unpublished.

Griffith, D. V. and P. A. Lane (1999) Slope StabilityAnalysis by Finite Elements. Geotechnique 49,No. 3., p 387–403

Hartono, U., Baharuddin, dan Brata, K. (1992) Petageologi regional lembar Madiun. Pusat Penelitiandan Pengembangan Geologi.

Hammah, R.E, Curran, J.H., Yacoub, T., andCorkum, B. (2004) Stability Analysis of RockSlopes using the Finite Element Method. In: EU-ROCK 2004 & 53rd Geomechanics Colloquium,Schubert (ed.).

Hoek, E., Carter, T.G., Diederichs, M.S. (2013) Quan-tification of the Geological Strength Index Chart.Geomechanics Symposium, USA.

National Earthquake Research Centre (2017) PetaSumber Bahaya dan Gempa Indonesia, Jakarta.Kementerian Pekerjaan Umum dan PerumahanRakyat.

Pulunggono, A. dan Martodjojo, S. (1994) Peruba-han Tektonik Paleogen-Neogen di Jawa. In: Proc.Seminar Geologi dan Geotektonik Pulau Jawa.Yogyakarta. Geological Engineering DepartmentGadjah Mada University.

Rocscience Inc. (2001) Application of the finiteelement method to slope stability, Toronto, 2001–2004. URL: https://www.rocscience.com/assets/resources/learning/papers/Application-of-the-Finite-Element-Method-to-Slope-Stability.pdf

Wyllie, D.C. (2018) Rock Slope Engineering–CivilApplications. Taylor & Francis Group, LLC.


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Numerical Evaluation of Tunnel Portal Slope Stability at